1. Field
The invention is in the field of ammonium nitrate-fuel oil based slurry explosives.
2. State of the Art
Explosives are an important part of the mining industry and it is estimated that over three billion pounds of explosives are used each year in that industry in the United States alone. The most commonly used basic explosive is a mixture of ammonium nitrate and fuel oil commonly referred to as ANFO. This generally comprises 94% ammonium nitrate mixed with 6% No. 2 fuel oil. It is a dry explosive and is generally marketed in bags or in bulk. It is not water resistant so cannot be used in damp or water immersed environments.
Over the years many attempts have been made to increase the explosive power of a given amount of ANFO, to make it water resistant so that it can be used in damp and water immersed environments such as in blast holes containing water, and to make it easier to handle and load into bore holes for blasting. A common way of increasing the explosive strength of the ANFO is by the addition of a metal powder such as aluminum powder. A way of making the explosive easier to handle in the filling of bore holes is to make the explosive in the form of a flowable slurry, and slurry explosives have become very popular. By thickening the slurry in various ways, a slurry can be made to have varying degrees of water resistance.
The term strength of an explosive refers to the energy content of the explosive, which in turn, is a measure of force or power it can develop and its ability to do work. The term strength is used with ammonium nitrate based explosives to compare their energies to ANFO and is measured in kilo-calories per gram. In the case of ANFO, which is the base of reference, the value of weight strength is 1.0 k-cal/gm. The other term, bulk strength, compares explosives on a bulk or volume basis, and is a function of the density of the material. The higher the density, the higher the bulk strength. Its unit of measure is kilo-calories per cubic centimeter, abbreviated k-cal/cc.
The most common form of slurry explosives are emulsions which are mixed with ANFO. The emulsions are oil in water or water in oil emulsions which contain oxidizers and fuels in various proportions and are mixed with ANFO wherein the emulsion generally makes up 30% to 50% of the final mixture. Emulsion explosives generally have less weight strength then ANFO, but usually have increased density.
The performance of emulsion-ANFO products is erratic. Not all are water-resistant and the shelf life of bagged material is notoriously poor. Emulsions are basically unstable and tend to separate. Many mines and quarries have experienced shot failures. The density of the final emulsion-ANFO product is not what one would call a natural density but is often controlled by the addition of perlite or glass microspheres. These things are inert materials and in an explosive reaction, do not add to the energy, but subtract from it.
Another class of slurry explosive is known as water gels. These are combinations of various oxidizers, fuels and other chemicals, including water, with the major constituent being ammonium nitrate. Water gels are marketed in bags or in bulk and are water resistant. Water gels can be cross-linked, i.e., can be made more viscous, water resistant or self-supporting by chemical addition.
Conventional water gels are complicated and costly to make. A well known example is Thermex's T-600. About one half of the ingredients are made into a mother liquor and this requires substantial heating. The rest of the dry ingredients are then blended with the mother liquor in a ribbon mixer or a large concrete mixer. The ingredients are water, ammonium nitrate, hexamine, nitric acid, ammonium perchlorate, gilsonite and sodium nitrate. The materials used are expensive compared to ANFO as is the plant equipment to make the water gel, and such gels are not made on site.
There remains a need for an inexpensive, easily made slurry explosive having increased explosive strength over ANFO and which is highly water resistant.